Method of speed increasing photopolymerizable compositions



United States Patent 3,183,094 METHOD OF SPEED INCREASING PHOTO- POLYMERIZABLE COMPOSITIONS Edward Cerwonka and Andre K. Schwerin, Binghamton, 1 N.Y., assignors to General Aniline & Film Corporation,

New York, N.Y., a corporation of Delaware No Drawing; Filed Aug. 10, 1960, Ser. No. 48,569

7 Claims. (Cl. 96-92) The present invention relates to ferric salt-pcrcompounds tor catalyzing the photopolymerization of ethylenically unsaturated monomers and, more particularly, to the utilization in such system of a reducing compound reactive with hydrogen peroxide and capable of greatlyincreasing the speed of the photopolymerization procedure.

In application Serial No. 808,882, filed April 27, 1959, by Helene D. Evans et al., entitled Photopolymerization of Unsaturated Organic Compounds by Means of Radiation Sensitive Iron Compounds as Photoinitiators, now US. Patent 3,101,270, dated August 20, 1963, it is indicated that the ferric salts in the presence of a pcrcompound are capable of greatly facilitating the photopolymerization of unsaturated organic compounds in the presence of UV. or white light either in bulk form or in the form of coatings. While the speed of the system envisaged in the patent is quite high compared to photopolymerization utilizing only organic peroxides such as benzoyl peroxide, there is considerable room for increasing the speed, particularly insofar as photopolymerizable coatings are concerned.

Typically, in Example IV of the patent, a coating was made from gelatin, a'crylamide, N,N'-methylene-bisacrylamide, water, ferric ammonium citrate and a wetting agent such as sodium lauryl sulfate. After coating on a paper base and drying, the coating was exposed for 30 seconds through a silver negative using a 375 watt tungsten lamp at a distance of 12". Subsequent to exposure, the coating was treated with a 1% by weight solution of hydrogen peroxide. After washing with water, a polymerizable resist remained in the exposed areas.

Thirty seconds, manifestly, is an extended period to which to subject a photographic coating to light. Thus, the usual silver halide coatings when exposed at camera speeds, i.e.,fractions of a second, yield dense images.

The photopolymerizable coatings of the application are,

therefore, unsuitable for use at photographic speeds.

' In the method of the patent, the coating may be made on film, paper, metal or similar base. Preferably, a cross-linking agent is included in the coating with the ferric salt. The exposure to visible light yields a quantity of ferrous ions depending on the intensity of the exposure. Subsequent passage of the coatings through an aqueous solution of a percompound having the grouping -O--O-, such as a peroxide, persulfate or perborate, produces after washout of unexposed areas a relief type of polymer.

Any given coating will have a standard exposure speed by which is meant the minimum exposure time with light of constant intensity to give a perceptible photoresist. An alternative method of determining speed is exposure of a coated base through a filmbase stepwedge. The number of steps obtained upon washout gives an indication of relative speed.

For many purposes, as previously indicated,-it is desirable to increase the speed of a given iron salt-mono mer-percompound system. One method that suggests itself is that of increasing the rate of photoreduction of ferric-to-ferrous ions with a light source of constant light intensity. Typically, ferric ammonium oxalate shows faster photoreduction of the ferric ion than ferric am- 3,183,094 Patented May 11, 1965 m onium citrate due to its higher absorption coellicient. Another method of increasing speed consists of acceleratmg the secondary reactions even though the rate of photoreduction of ferric-to-ferrous ions may be unchanged. In this method, there is an increase in the rate of polymeriz ation which takes place when the coating is treated with a percompound.

It is now discovered that the speed of photopolymerization of ethylenically unsaturated monomers by way of the ferric salt-percompound system is greatly augmented by the utilization of a reducing agent readily reactive with hydrogen peroxide and which is oxidized by a chain reaction in the presence of a ferrous salt and hydrogen peroxide. 1

The increase in speed of the photopolymerization systems employing a ferric salt and a percompound by resort to such speed increasing reducing agents and coatings for such systems constitute the purposes and objects of our invention.

The compounds contemplated for use as speed-increasing reducing agents may be inorganic or organic. Among the inorganic compounds are the oxy acids of sulfur or their salts and nitrogen compounds such as hydrazine and hydroxylamine which react with peroxides to liberate hydroxyl radicals. Illustrative of such compounds are sulfurous acid, sodium sulfite, sodium bisulfite, sodium hydrosulfite, sodium thiosulfate, hydrazine, hydroxylamine and the like (see also Trans. Far. Soc. 42, page (1946)).

Organic compounds suitable for use are alcohols, hydroxy acids, ethers, ,aldehydes, amino acids and thercapto carboxy acids.. Illustrative of such compounds are cetyl alcohol, glyoxylic acid, glycollic acid, glycolo'side, glyoxal, the addition product of glyoxalwith sodium'bisulfite, I glycine, phenyl glycine, mercapto acetic acid, mercapto propionic acid, sugars, i.e., glucose, maltose, and the like. These speed-increasing agents form carbon, oxygen, nitrogen or sulfur radicals in which the electron density at the point of free valency is relatively high. The value of the reduction potential falls there after within certain limits.

The quantity of the speed-increasing agent vis-a-vis by weight of the monomer to 1 part by weight of th speed-increasing agent.

The radiation-sensitive ferric compounds utilizable in our system are thosedescribed in the aforesaid US. Patent 3,101,270. Examples of such compounds are ferric acetate, ferric ammonium acetate (brown), ferric ammonium citrate (green), ferric ammonium oxalate, ferric ammonium sulfate, ferric ammonium tartrate, ferric bromide, ferric chloride, ferric citrate, ferric formate, ferric glycerol phosphate, ferric hydroxide, ferric nitrate, ferric phosphate, ferric potassium citrate, ferric potassium tartrate, ferric pyrophosphate, ferric sodium oxalate, ferric subsulfate, ferric sulfate, ferric succinate and the like. 4

As a source of free radicals we prefer pcrcompounds containing the grouping -OO. Examples of such pcrcompounds are: hydroperoxides such as hydrogen peroxide, aliphatic hydroperoxides, i.e., methyl hydroperoxide, ethyl hydroperoxide, t-butyl-hydro-peroxide, hexyl hydroperoxide, octyl .hydroperoxide, trans-decalin hydroperoxide, l-mcthylcyclopentyl hydroperoxide, 1,1-dimethyl-2-propenyl hydroperoxide, Z-cyclohexene-Lyl hydroperoxidc, cumene hydroperoxide, tetralin hydroperoxide, triphenyl methyl hydroperoxide, etc.; peroxides of the formula ROOR' wherein R and R, which may or may not be alike can be-alkyl such as methyl, ethyl, propyl, butyl, pentyl, hexyl,'heptyl, octyl, nonyl, decyl, undecy], etc.; aralkyl, i.e., benzyl, phenethyl, phenylpropyl, naphthylmethyl, naphthylethyl, naphthylpropyl, etc.; aryl such as phenyl, naphthyl, etc.; aliphatic acyl such as acetyl, propionyl, butyryl, valeryl, etc.; aromatic aeyl such as benzoyl, naphthoyl, etc.; peroxy acids, i.e., aliphatic peroxy acids, e.g., peracetic acid, perpropionic acid, perbutyric acid, etc.; aromatic peroxy acids, i.e., perbenzoie acid, perphthalic acid, etc.; esters of the aforesaid peroxy acids; salts of peracids such as ammonium persulfate, etc. Such percompounds are well known and their description and preparation can be found in the chemical literature. In this connection, reference is made to such well known works as Organic Peroxides," by Arthur V. Tobolsky and Robert B. Mesrobian and published by Interscience Publishers, Inc., New York, and Interscience Publishers Ltd., London (1954).

Any normally liquid or solid photopolymerizable ethlenically unsaturated monomer is applicable in the practice of our invention. Compounds particularly advantageous are the photopolymerizable vinyl or vinylidene compounds containing a CH =C group activated by direct attachment to a negative group such as halogen, k0, CEN, C-=-C, -O-, or aryl. Examples of such photopolymerizable unsaturated organic compounds include acrylamide, acrylonitrile, N-ethanol acrylamide, methacrylic acid, acrylic acid, calcium acrylate, methacrylamide, vinyl acetate, methylmethacrylate, methlacrylate, ethylacrylate, vinyl benzoate, vinyl pyrrolidone, vinylmethyl ether, vinylbutyl ether, vinylisopropyl ether, vinylisobutyl ether, vinylbutyrate, butadiene or mixtures of ethylacrylate with vinyl acetate, acrylonitrile with styrene, butadiene with acrylonitrile and the like.

The above ethylenically unsaturated organic compounds, or monomers may be used either alone or in admixture in order to vary the physical properties such as molecular weight, hadness, etc. of the final polymer. Thus, it is recognized practice, in order to produce a vinyl poylmer of the desired physical properties, to polymerize in the presence of a small amount of an unsaturated compound containing at least two terminal vinyl groups each linked to a carbon atom in a straight chain or in a ring. The function of such compounds is to cross-link the polyvinyl chains. This technique, as used in polymerization, is further described by Kropa and Bradley in vol. 31, No. 12, of Industrial and Engineering Chemistry, 1939. Among such cross-linking agents for the purpose described herein may be mentioned N,N'-methylene-bis-acrylamide, triallyl cyanurate, divinyl benzene, divinyl ketones and diglycol-diacrylate. Generally speaking, increasing the quantity of cross-linking agents increases the hardness of the polymer obtained in the range wherein the ratio of monomer to cross-linking agent varies from 10:1 to 50:1.

The quantity of ferric salt used to initiate polymerization of the monomer or unsaturated organic compound is not critical and may be varied over wide limits. In general, we have found that satisfactory results ensue if the proportion of ferric ion to monomer varies from 1110,000 to 1:6.

The photopolymerization can be carried out under a wide variety of conditions employing numerous modifications. Our system of photopolymerization is particularly valuable in the field of photography where its high speed and response to relatively low intensity radiation sources make it ideal for imagewise polymerization.

One valuable application of our process, for example, is the production of relief printing plates for use in the graphic arts. Such plates can be fabricated by coating a mixture of monomer or monomers in a suitable solvent plus a small quantity of ferric salt, and speed-increasing reducing agent on a suitable base or support. The resulting coating can then be exposed to a radiation source such as an incandescent bulb having a tungsten filament after which it is treated with a percompound. The exposed areas undergo rapid polymerization in the presence of the percompound, speed increasing reducing agent and irradiated ferric salts, whereas the unexposed areas are left unaffected so that the unreacted or unpolymerized monomers may be washed off or otherwise removed. A resist is thus formed of the photopolymerized polymer which can be used as a negative working relief plate. By employing a hydrophilic surface as the support for the coating, such as a partially saponified cellulose acetate, a plate is produced having wash-out resistant areas. Such a plate can then be transformed by well known means into a negative working offset plate.

In the field of photography, our invention can be used, for example, for the production of black and white prints. Thus, a light-sensitive plate is prepared having coated thereon a layer comprising a monomer, a light-sensitive ferric compound, speed increasing reducing agent and a finely divided black pigment dispersed in the coating. This plate is then exposed beneath a silver negative which causes polymerization in the exposed areas of the coating when processed with a percompound. After washing with water to remove unpolymerized monomer in the unexposed regions, there is obtained a reversed polymeric photographic image.

In most instances, where photographic coatings are used, it is desirable to employ a normally solid hydrophilic colloid as the carrier for the light-sensitive ferric compound. Suitable colloid carriers for this purpose are those used in photography and include polyvinyl alcohol, casein, glue, saponified cellulose acetate, carboxymethyl cellulose, starch, gelatin, and the like.

Another photographic application of our invention is in color reproduction. For example, a light-sensitive plate is prepared as described above, i.e., containing a monomer, ferric compound, and speed-increasing reducing agent, and exposed to one of the primary color aspects of a subject using a color separation negative. After treatment with a percompound to effect polymerization in the exposed areas, the so obtained polymerized image is then subtractively dyed. By exposing other light-sensitive plates to the remaining primary color aspects of the subject processing as described above to effect polymerization in the exposed areas, and subsequently dyeing with the appropriate subtractive dye, superimposition of the resulting subtractively colored images will then reproduce the original subject.

Other uses to which the above photopolymers may be put include such photographic and lithographic applications as, for example, in the production of bimetallic printing plates, etched copper half-tone images, printing plates having cellulose ester supports, grained zinc or aluminum lithographic plates, zincated lithographic printing plates, ungrained copper printing plates for preproofing copper chromium bimetallic plates, etc.

Numerous materials are suitable as supports or bases for the radiation-sensitive plates prepared in accordance with the process described herein, such as cellulose ester supports including the hydrophobic variety or the type having a surface rendered hydrophilic by a partial saponification, metals such as aluminum or zinc, polyethylene terephthalate polymers, paper, glass, polystyrene, polycarbonates, etc.

A further advantage of our photosensitive coatings and materials arises as a result of their stability so that they are not adversely affected on storage under conditions of excessive humidity and temperature. In this respect, the new materials are superior to the old bichromated glue or albumin layers which must be prepared and sensitized just prior to usage because of their poor keeping qualities.

As previously pointed out, it is a distinct advantage of the invention that the exciting or exposing radiation can be from a low intensity source such as an ordinary household incandescent lamp. Thus, the use of high energy radiation sources such as carbon flame arcs or mercury arcs, commonly employed to effect photopolymerization in the prior art processes, are dispensed with in our process. Furthermore, our photosensitive materials, in addition to eliminating the need for high intensity radiation sources, also possess high speed requiring fractions of a second to a few seconds depending on light conditions to effect imagewise polymerization and are thus equal to some of the lower speed silver halide photographic products. Such a characteristic suggests that the mechanism by which the process operates is of the free radical type, involving a redox system or elimination of oxygen which is detrimental to photopolymerization. When the iron compounds in the form of ferric salts are exposed to light, they are transformed or reduced to the ferrous state which, in the presence of our speed-increasing reducing agent and percompound results in the formation of free radicals which, in turn, causes polymerization of the monomers. Furthermore, only a few photoproduced ferrous ions are required to initiate rapid and vigorous polymerization in the presence of the speed-increasing reducing agent and percompound since very low exposure times to relatively feeble radiation are sufficient. In other words, the photopolymerization of the instant invention makes use of an amplification factor, that is to say, slight photochemical changes can be used to trigger or modulate a much more vigorous secondary reaction, in this case, the polymerization of the aforesaid monomers.

In commenting on photographic coatings we have mentioned mainly'the incorporation therein of the colloidal carrier, monomers, speed-increasing reducing agent and ferric salt with development after exposure with an aqueous percompound. The speed-increasing agent may be omitted from the coating and added to the aqueous percompound radical producer shortly before application of the developer solution. In either of these alternate procedures, the speed is approximately the same. However, it is preferable to combine in the coating, the monomer, sensitive ferric compound and speed-increasing reducing agent followed by exposure and subsequent treatment with the percompound. The latter procedure has the advantage of yielding an extremely stable composition from which coatings can be manufactured having very long shelf life both before and after exposure. Since, in this ramification, the percompound does not contact the iron sensitized monomer until after exposure, a highly reactive type of percompound can be employed as the radical producer to bring about rapid polymerization. In other words, a latent image of irradiated or exposed ferric salts in the presence of the speed increasing agent is used to trigger or effect very rapid imagewise polymerization of an unsaturated organic compound.

It is understood that the critical feature of our invention resides in the speed increasing reducing agent. A list of these has already been given and certain have been referred to in the literature such as J.II. Merz and W. A. Waters, Journal of the Chemical Society, S -15 (1949). The specific compounds involved are not the only compounds that may be used. On the contrary, it is a simple matter for anyone skilled in the art to determine if a compound is utilizable by treating it with ferrous salt and hydrogen peroxide and noting the rate at which the reducing agent is oxidized by the Fentons reagent. Alternatively, it is a simple expedient to determine if a coating containing the monomer, ferric salt and speed-increasing reducing agent gives a dark reaction or if the selected speed increasing reducing agent turns out to be a polymerization inhibitor.

It may be added at this point that by such tests we have ascertained that some organic reducing agents are incapable of functioning to increase the speed of photopolymerization. Typically, trials with p-toluene sulfinic acid sodium salt, hydroquinone, ascorbic acid and allyl thiourea are ineffective. This may be attributable to the fact that the compounds cause dark polymerization (phenyl hydrazine hydrochloride) or are polymerization inhibitors (i.e., hydroquinone). However, the sheep can readily be separated from the goats by the tests previously referred to.

Example I A contro or type coating was prepared by coating on a paper base the following solution:

Gelatin "grams": 5 Water, de-ionized milliliters 40 A6 (approximately 60% acrylamide, 2.33% N,N-

methylene bis-acrylamide in water) milliliters 6 Glycerin drops 15 Feric ammonium citrate (brown) (36 grams/ milliliters of de-ionized water) milliliters 5 A sample strip was exposed through a photoprint negative and through a neutral density filter having an optical density of 1.0 to a 375 watt photorefiector lamp at sixteen inches. The exposed. coating was processed in a 1% aqueous solution of hydrogen peroxide and washed with water to remove unpolymerized monomer. The minimum time required to give a photoprint resist was found to be fifteen seconds.

Example II A solution was prepared by adding to the type-or control formulation the following:

Grams Disodium Versene (disodium dihydrate of ethylene diamine tetra/ acetic acid) 0.50 Sodium sulfite, anhydrous 0.50

When this preparation had been coated on paper base, and dried, a sample strip was exposed and processed in the manner described for the control coating. The minimum exposure time through a photoprint and neutral density filter (O.D.=l.0) was found to be less than two seconds. Replacing the neutral density filter (O.D.=l.0) with one of ten times its absorption (O.D.=2.0). The minimum exposure time was found to be fifteen seconds. A control coating exposed and processed in this manner gave no image. The speed of the accelerated coating is, therefore, about ten times that of the control.

Example 111 A solution was prepared from the formulation given for the control" coating, to which were added:

Grams Sodium bisulfite 0.50 Disodium Versene? 0.50

Exposure and processing of this coating in the manner described for the control showed the same increase in speed as was observed with the coating of Example II.

Example IV A coating was prepared by coating on paper the formulation given for the control coating to which were added:

Grams Sodium hydrosulfite 0.50 Disodium Versene 0.50

Exposure of a test strip and processing as in Example I showed a photoresist to be produced in ten seconds through a photoprint negative and through a neutral density (O.D.=2.0) filter. The increase in speed was, therefore, greater than that in Examples II and III.

To avoid 7 Example V To the control coating formulation were added:

Grams Sodium thiosulfate 0.50 Disodium Versene 0.50

A test strip of the above coating exposed and processed in the same manner as the contro" test srip showed production of a resist in five seconds exposure. Speed had therefore been increased by a factor of three.

Example VI To the control coating formulation, there was added the following reagent:

Glyoxylic acid gram 0.5

Exposure and processing of a strip of this coating in the same manner as the control showed that speed had been increased by a factor of 1.4.

Example VII To the control coating formulation there was added the following reagent:

Glyoxal-sodium bisulfite addition compound dihydrate gram 0.5

Exposure and processing of a strip of this coating in the same manner as the control showed that speed had been increased by a factor of 1.4.

Example VIII A coating was prepared containing in addition to the materials used in formulating the control the following component:

Glycine gram.. 1.0

Exposure and processing of a strip of this coating in the same manner as the control showed that speed had been increased by a factor of 4.

Example IX A coating was prepared by adding the following to the formulation of the control:

Mercapto-acetic acid sodium salt gram 0.5

Exposure and processing of a strip of this coating in the same manner as the control" showed that speed had been increased by a factor of 1.4.

Example X A composition was prepared according to the control coating of Example I, coated on a paper base and dried. A sample strip was exposed through a photoprint negative and through a neutral density filter having an optical density of 1 to a 375 watt photorefiector lamp at 16". The exposed coating was processed in a aqueous solution of hydrogen peroxide containing .25 gram of sodium thiosulfate. The results were similar to those of Example V.

We claim:

1. A process of producing a polymeric photographic image which comprises irradiating, under a pattern, to

, provide for the conjoint presence image-wise in said coating of the monomer, ferrou ions, peroxide and speedincreasing reducing agent and causing image-wise photopolymerization of the monomer through the agency of the ferrous ions and peroxide while augmenting the rate of' phot-opolymerization by the formation of hydroxyl radi-- cals through the agency of the peroxide and said speedincreasing reducing agent.

2. The process as defined in claim 1 wherein the photo-- polymerization speed-increasing reducing agent is hydrazine.

3. The process as defined in claim 1 wherein a chelating agent is present in the colloidal carrier, said chelating agent being capable of complexing traces of heavy metal ions.

4. The process as defined in claim 1 wherein the photopollgmerization speed-increasing reducing agent is sodium su te.

5. The process as defined in claim 1 wherein the radiation is effected with visible light.

6. The process as defined in claim 1 wherein the photopolymerization speed-increasing reducing agent is hydroxylamine.

7. The process as defined in claim 3 wherein the chelating agent is disodium dihydrate of ethylene diamine tetra acetic acid.

References Cited by the Examiner UNITED STATES PATENTS 2,344,785 3/44 Owens et a1. 96-115 2,760,863 8/56 Plambeck 96-115 X 2,875,047 2/59 Oster 96--35 3,029,145 4/62 Dumers et a1 96-35 3,038,800 6/62 Luckey et a1. 96115 3,041,172 6/62 Evans et a1. 96-115 3,065,160 11/62 Levinos et al. 96-115 3,101,270 8/63 Evans et a1 96--1l5 OTHER REFERENCES Glafkides: Photographic Chemistry, Fountain Press, London, 1958, vol. I, p. 83.

Organic Chemistry, Decering, Barnes & Noble, Inc., New York, College Outline Series, 6th Edition, 1951, p. 77.

NORMAN G. TORCHIN, Primary Examiner.

MILTON STERMAN, PHILIP E. MANGAN,

Examiners. 

1. A PROCESS OF PRODUCING A POLYMERIC PHOTOGRAPHIC IMAGE WHICH COMPRISES IRRADIATING, UNDER A PATTERN, TO LIGHT RAYS RANGING FROM U.V. TO WHITE LIGHT, A COATING OF A COLLOIDAL CARRIER CONTAINING AN ETHYLENICALLY UNSATURATED MONOMER HAVING THE TERMINAL GROUPS CH2=C< ACTIVATED BY DIRECT ATTACHMENT TO A NEGATIVE GROUP, A PHOTOPOLYMERIZATION SPEED-INCREASING REDUCING AGENT WHICH IS READILY OXIDIZED BY PEROXIDES TO LIBERATE HYDROXYL RADICALS AND WHICH IS SELECTED FROM THE CLASS CONSISTING OF SULFUROUS ACID, SODIUM SULFITE, SODIUM BISULFITE, SODIUM HYDROSULFITE, SODIUM THIOSULFATE, HYDRAZINE AND HYDROXYLAMINE, AND A RADIATION-SENSITIVE FERRIC SALT DESIGNED TO PRODUCE FERROUS IONS IMAGE-WISE BY SUCH IRRADIATION AND TREATING THE IRRADIATED COATING WITH AN AQUEOUS SOLUTION OF A PERIOXIDE TO PROVIDE FOR THE CONJOINT PRESENCE IMAGE-WISE IN SAID COATING OF THE MONOMER, FERROUS ION,S PEROXIDE AND SPEEDINCREASING REDUCING AGENT AND CAUSING IMAGE-WISE PHOTOPOLYMERIZATION OF THE MONOMER THROUGH THE AGENCY OF THE FERROUS IONS AND PEROXIDE WHILE AUGMENTING THE RATE OF PHOTOPOLYMERIZATION OF THE FORMATION OF HYDROXYL RADICALS THROUGH THE AGENCY OF THE PEROXIDE AND SAID SPEEDINCREASING REDUCING AGENT. 